Parasites in the phylum apicomplexa contain a novel organelle, which appears to be a non-photosynthetic remnant of a plastid. Inhibitor studies in two apicomplexans, Toxoplasma gondii and Plasmodium falciparum; suggest that the plastid is essential for the growth of these parasites. Two functions identified in the apicomplexan plastid are fatty acid and isoprenoid biosynthesis. These functions require nuclear-encoded plastid proteins. Preliminary studies demonstrated that the pre-sequence of two nuclear-encoded plastid proteins is both necessary and sufficient to target to the T. gondii plastid. The long-term goal of the proposed study is to identify new targets for chemotherapeutic intervention in disease caused by apicomplexan parasites. It is hypothesized that nuclear-encoded T. gondii proteins are targeted to the plastid by a pathway similar to that used by algae with complex plastids. Three aspects of this hypothesis will be tested. First, we will determine if nuclear-encoded T. gondii proteins are targeted to the plastid directly from the endoplasmic reticulum or by trafficking through the Golgi. Second, we will determine if the signals that direst direct import through the T. gondii plastid membranes are conserved with respect to those used by higher plants. Third, we will determine if the accumulation of a partially-processed plastid pre-protein represents a rate limiting step in the targeting pathway. It is also hypothesized that the transit-like domain of the plastid pre-protein contains essential features required for targeting from the endoplasmic reticulum to the plastid. This hypothesis will be tested by in vitro mutagenesis of a plastid protein pre-sequence, transfection of the mutated reporter constructs into T. gondii and characterization of their localization and maturation in vivo.